Prenatal development of retinogeniculate axons in the macaque monkey during segregation of binocular inputs

Retinal ganglion cell circuits and glial interactions in humans and mice

Retinal ganglion cells (RGCs) are the brain's gateway for vision, and their degeneration underlies several blinding diseases. RGCs interact with other neuronal cell types, microglia, and astrocytes in the retina and in the brain. Much knowledge has been gained about RGCs and glia from mice and other model organisms, often with the assumption that certain aspects of their biology may be conserved in humans. However, RGCs vary considerably between species, which could affect how they interact with their neuronal and glial partners. This review details which RGC and glial features are conserved between mice, humans, and primates, and which differ. We also discuss experimental approaches for studying human and primate RGCs. These strategies will help to bridge the gap between rodent and human RGC studies and increase study translatability to guide future therapeutic strategies.

Refinement of the Primate Corticospinal Pathway During Prenatal Development

Perturbation of the developmental refinement of the corticospinal (CS) pathway leads to motor disorders. While non-primate developmental refinement is well documented, in primates invasive investigations of the developing CS pathway have been confined to neonatal and postnatal stages when refinement is relatively modest. Here, we investigated the developmental changes in the distribution of CS projection neurons in cynomolgus monkey (Macaca fascicularis). Injections of retrograde tracer at cervical levels of the spinal cord at embryonic day (E) 95 and E105 show that: (i) areal distribution of back-labeled neurons is more extensive than in the neonate and dense labeling is found in prefrontal, limbic, temporal, and occipital cortex; (ii) distributions of contralateral and ipsilateral projecting CS neurons are comparable in terms of location and numbers of labeled neurons, in contrast to the adult where the contralateral projection is an order of magnitude higher than the ipsilateral projection. Findings from one largely restricted injection suggest a hitherto unsuspected early innervation of the gray matter. In the fetus there was in addition dense labeling in the central nucleus of the amygdala, the hypothalamus, the subthalamic nucleus, and the adjacent region of the zona incerta, subcortical structures with only minor projections in the adult control.

Refinement of the retinogeniculate synapse by bouton clustering

Mammalian sensory circuits become refined over development in an activity-dependent manner. Retinal ganglion cell (RGC) axons from each eye first map to their target in the geniculate and then segregate into eye-specific layers by the removal and addition of axon branches. Once segregation is complete, robust functional remodeling continues as the number of afferent inputs to each geniculate neuron decreases from many to a few. It is widely assumed that large-scale axon retraction underlies this later phase of circuit refinement. On the contrary, RGC axons remain stable during functional pruning. Instead, presynaptic boutons grow in size and cluster during this process. Moreover, they exhibit dynamic spatial reorganization in response to sensory experience. Surprisingly, axon complexity decreases only after the completion of the thalamic critical period. Therefore, dynamic bouton redistribution along a broad axon backbone represents an unappreciated form of plasticity underlying developmental wiring and rewiring in the CNS.

Microglia sculpt postnatal neural circuits in an activity and complement-dependent manner

Microglia are the resident CNS immune cells and active surveyors of the extracellular environment. While past work has focused on the role of these cells during disease, recent imaging studies reveal dynamic interactions between microglia and synaptic elements in the healthy brain. Despite these intriguing observations, the precise function of microglia at remodeling synapses and the mechanisms that underlie microglia-synapse interactions remain elusive. In the current study, we demonstrate a role for microglia in activity-dependent synaptic pruning in the postnatal retinogeniculate system. We show that microglia engulf presynaptic inputs during peak retinogeniculate pruning and that engulfment is dependent upon neural activity and the microglia-specific phagocytic signaling pathway, complement receptor 3(CR3)/C3. Furthermore, disrupting microglia-specific CR3/C3 signaling resulted in sustained deficits in synaptic connectivity. These results define a role for microglia during postnatal development and identify underlying mechanisms by which microglia engulf and remodel developing synapses.

Wiring and rewiring of the retinogeniculate synapse

The formation and refinement of synaptic circuits are areas of research that have fascinated neurobiologists for decades. A recurrent theme seen at many CNS synapses is that neuronal connections are at first imprecise, but refine and can be rearranged with time or with experience. Today, with the advent of new technologies to map and monitor neuronal circuits, it is worthwhile to revisit a powerful experimental model for examining the development and plasticity of synaptic circuits--the retinogeniculate synapse.

Learning the Gestalt Rule of Collinearity from Object Motion

The Gestalt principle of collinearity (and curvilinearity) is widely regarded as being mediated by the long-range connection structure in primary visual cortex. We review the neurophysiological and psychophysical literature to argue that these connections are developed from visual experience after birth, relying on coherent object motion. We then present a neural network model that learns these connections in an unsupervised Hebbian fashion with input from real camera sequences. The model uses spatiotemporal retinal filtering, which is very sensitive to changes in the visual input. We show that it is crucial for successful learning to use the correlation of the transient responses instead of the sustained ones. As a consequence, learning works best with video sequences of moving objects. The model addresses a special case of the fundamental question of what represents the necessary a priori knowledge the brain is equipped with at birth so that the self-organized process of structuring by experience can be successful.

Development of visual projections follows an avian/mammalian-like sequence in the lizard Ctenophorus ornatus

Development of primary visual projections was examined in a lizard Ctenophorus ornatus by anterograde and retrograde tracing with DiI and by GAP-43 immunohistochemistry. Visual pathway development was essentially similar to that in birds and mammals and thus differed from patterns in fish or amphibians. A number of features characterised the development as mammalian-like. Three phases occurred in rapid succession after laying: outgrowth (2-3 weeks, early), exuberance (4-5 weeks, intermediate), and retraction to the adult pattern (6-8 weeks, late) at about the time of hatching and eye opening. Furthermore, ipsilateral projections developed with only a slight lag relative to the contralateral ones. The dorsally located fovea could be identified from early stages. Optic axons formed transient exuberant projections to the ipsilateral optic tectum, to the opposite optic nerve, and to nonvisual regions. The pattern resembled that formed in the long term by regenerating optic axons in C. ornatus (Dunlop et al. [2000b] J. Comp. Neurol. 416:188-200), suggesting that axons recognise molecular signals associated with the initial exuberant innervation but not those associated with subsequent refinement.

Development of individual axon arbors in a thalamocortical circuit necessary for song learning in zebra finches

Individual axon arbors within developing neural circuits are remodeled during restricted sensitive periods, leading to the emergence of precise patterns of connectivity and specialized adaptive behaviors. In male zebra finches, the circuit connecting the medial dorsolateral nucleus of the thalamus (DLM) and its cortical target, the lateral magnocellular nucleus of the anterior neostriatum (lMAN), is crucial for the acquisition of a normal vocal pattern during the sensitive period for song learning. The shell subregion of lMAN as well as the entire terminal field of DLM axons within lMAN undergo a striking increase in overall volume during early stages of vocal learning followed by an equally substantial decrease by adulthood, by which time birds have acquired stable song patterns. Because the total number of DLM neurons remains stable throughout this period, the dramatic changes within the overall DLM-->lMAN circuit are presumably attributable to dynamic rearrangements at the level of individual DLM axon arbors over the course of vocal learning. To study such rearrangements directly, we reconstructed individual DLM axon arbors in three dimensions at different stages during vocal learning. Unlike axon arbors in other model systems, in which the number of branches increases during development, DLM arbors are unusual in that they have the greatest number of branches at the onset of vocal learning and undergo large-scale retraction during the sensitive period for song learning. Decreases in the degree of overlap between DLM arbors apparently contribute to the increased overall volume of the DLM-->lMAN circuit during vocal learning. These developmental changes in DLM axon arbors occur at the height of the sensitive period for vocal learning, and hence may represent either a morphological correlate of song learning or a necessary prerequisite for acquisition of song.

Independent parcellation of the embryonic visual cortex and thalamus revealed by combinatorial Eph/ephrin gene expression

The visual cortex in primates is parcellated into cytoarchitectonically, physiologically, and connectionally distinct areas: the striate cortex (V1) and the extrastriate cortex, consisting of V2 and numerous higher association areas [1]. The innervation of distinct visual cortical areas by the thalamus is especially segregated in primates, such that the lateral geniculate (LG) nucleus specifically innervates striate cortex, whereas pulvinar projections are confined to extrastriate cortex [2--8]. The molecular bases for the parcellation of the visual cortex and thalamus, as well as the establishment of reciprocal connections between distinct compartments within these two structures, are largely unknown. Here, we show that prospective visual cortical areas and corresponding thalamic nuclei in the embryonic rhesus monkey (Macaca mulatta) can be defined by combinatorial expression of genes encoding Eph receptor tyrosine kinases and their ligands, the ephrins, prior to obvious cytoarchitectonic differentiation within the cortical plate and before the establishment of reciprocal connections between the cortical plate and thalamus. These results indicate that molecular patterns of presumptive visual compartments in both the cortex and thalamus can form independently of one another and suggest a role for EphA family members in both compartment formation and axon guidance within the visual thalamocortical system.

Molecular evidence for the early specification of presumptive functional domains in the embryonic primate cerebral cortex

To identify molecules that may play a role in the initiation of cerebral cortical area formation, we examined the expression of the Eph receptors and their ligands, the ephrins, during primate corticogenesis. We selected the macaque monkey neocortex because of its clear areal subdivisions, large surface area, protracted development (gestation = 165 d), and similarity to the human brain. In situ hybridizations, performed at early [embryonic day 65 (E65)], middle (E80), and late (E95) stages of cortical development, revealed that EphA system family members are expressed in distinct gradients and laminar and areal domains in the embryonic neocortex. Indeed, several regionally restricted molecular patterns are already apparent within the cortical plate at E65, before the formation of thalamocortical connections, suggesting that the initial expression of some EphA system members is regulated by programs intrinsic to cortical cells. For example, EphA3, EphA6, and EphA7 are all selectively expressed within the presumptive visual cortex. However, although EphA6 and EphA7 are present throughout this region, EphA3 is only expressed in the prospective extrastriate cortex, suggesting that cortical cells harbor functional biases that may influence the formation of appropriate synaptic connections. Although several patterns of early gene expression are stable (e.g., EphA3, EphA4, and EphA6), others change as development proceeds (e.g., EphA5, EphA7, ephrin-A2, ephrin-A3, and ephrin-A5), perhaps responding to extrinsic cues. Thus, at E95, after connections between the cortical plate and thalamus have formed, receptor subtypes EphA3, EphA5, EphA6, and EphA7 and the ligand ephrin-A5 are expressed in posterior regions, whereas EphA4 and ephrin-A2 and ephrin-A3 are either uniformly distributed or anteriorly biased. Taken together, our results demonstrate molecular distinctions among cells of the embryonic primate neocortex, revealing hitherto unrecognized compartmentalization early in corticogenesis.